In heat-sealing thermoplastic parts, as disclosed in CH-PS 532,996 and DE-PS 2,242,369, large temperature differentials in the parts being heat sealed should be avoided. Large temperature differentials are avoided to prevent development of superimposed and uncontrollable material stresses.
Two adjacent pipe ends can be connected by an electrically operated heat-sealing apparatus in the form of a sleeve or saddle member mounted on the pipes. The sleeve or saddle member must tightly contact surfaces of the pipes being connected, at least in the zones in which the desired heat-seal is formed. With sleeves, such as that disclosed in EP-A1-0036 963, the tight engagement with the pipes results from shrinking the sleeve. Shrinking of the sleeve is accomplished by forming inherent shrinkage stresses in the sleeve by the injection molding process or other manufacturing steps. The stresses are relieved when the sleeve is heated causing the sleeve to shrink tightly about the adjacent pipe surfaces. For saddle members, as in DE-PS 2,242,369, a mechanical force is applied to the exterior of the saddle members or on a bracket to compress the saddle members tightly about the pipes being connected.
The seam between the pipes being connected is subjected to a bearing load during the entire process when electrical energy is supplied to the heating element. The bearing load produces the fusion pressure required to provide a good heat-seal.
Advantageously, the parts to be connected are heated as uniformly as possible throughout their entire thicknesses in the seam area and across the entire surfaces to be joined. The uniform heating minimizes undesirable stresses caused by only partial heating. Heat-sealing sleeves with a heating coil passing through them, as in EP-A1-0036 963, can provide the desired uniform heating. Such sleeves can heat the pipe ends being joined through their entire thicknesses and along the entire length of the sleeve which defines the sealing zone.
The heat-shrinkable sleeve with a heating coil uniformly compresses and heats the connected parts, as well as minimizes stress differentials in the subsequently formed heat-seal.
Conventional systems for heat-sealing thermoplastic pipes are disadvantageous, particularly for relatively thin-walled pipes. Thin-walled pipe ends collapse or deform during the process such that insufficient fusion pressure is developed at the pipe ends. The lack of adequate pressure permits melted material to flow between the pipe ends and into the pipe interior forming a burr. The flow of melted material into the seam area is increased in systems where the heating wires are forced into the seam area applying concentrated heat energy to an undesirable degree in the connection. These disadvantages are further intensified with materials having a high melt index.
In other conventional systems (e.g., CH-A 553,368), the heat-sealing sleeves do not have heating coils in their central areas. The omission of a heating coil in the central area avoids heating the pipe ends to a significant degree. This prevents collapse and deformation of the thin-walled pipe ends such that melted material does not pass between the pipe ends. However, these systems produce non-uniform heating in the heat-sealing sleeve such that the sleeve cannot be properly shrunk-fitted in its central area. The non-uniform shrinking of the sleeve creates a high differential of internal stresses in the sleeve which adversely affects the life of the connection formed thereby.